Two scenarios here.
1. WhatsApp uses your phone contacts to generate its contact list if you have enabled the sync option between WA and your contacts. This may vary depending on the phone you use but I base this on the android phone I use.
2. You do not have the sync option enabled which means you may have lost the contacts you need.
Solution I can give you is that you install WA, restore the WA database which has messages from your "lost" contacts and see which numbers you want/need. The restore option happens when you first start to use WA.